The present invention is directed to apparatus for blanking electron beams in electron-beam instruments. Its primary, but not exclusive, application is to electron-beam stroboscopy.
The instrument most widely used to observe electrical signals is, of course, the oscilloscope. The user places the tip of an oscilloscope probe on the circuit node to be observed, and the probe signal is amplified and applied to cathode-ray-tube (CRT) deflection plates so that a visual representation of the signal is produced on the CRT screen. As circuits have become miniaturized, however, the conventional method of obtaining the signal--i.e., manual placement of a probe on a node--does not work for some nodes of interest, because the nodes are physically much too small.
The solution to this problem has been electron-beam stroboscopy. An electron-beam stroboscope uses much the same apparatus as an electron microscope does. Specifically, a beam of electrons is focused on a very small region of the object to be studied, in this case an operating integrated circuit. The secondary electrons emitted by the circuit are collected, and a signal representative of the secondary current is produced. In standard electron microscopy, an image is generated from the differences caused in the secondary current by the different physical features that the beam encounters as it is scanned over a region of the object.
In contrast, the electron beam in electron-beam stroboscopy is generated in short pulses whose time of occurrence is determined by reference to a trigger signal that represents a particular time in each cycle of a device under test. The magnitude of the emitted-electron flux depends on the voltage at the point in the device on which the beam is focused, so an image can be generated from the differences caused in the secondary current by the different voltages encountered by different pulses of the electron beam.
There are two major modes of electron-beam stroboscopy. They are distinguished from each other by the different relationships between successive pulses. In the first mode, which results in spacial scanning, successive pulses occur in the same time relationship to successive trigger signals but are deflected by different distances so that they hit different points on the device under test. This mode generates a voltage contour of the region of interest for a particular instant in the cycle of the device.
The second mode results in a plot of voltage versus time for a particular spacial point on the device. In this mode, the deflection remains the same for successive cycles, but each electron-beam pulse is spaced in time from its respective trigger signal by a different delay. In both modes, the electron-beam pulses must last for very short time intervals because high-speed integrated circuits present the need to observe voltages that change very quickly.
These short intervals are difficult to achieve. One of the factors that has limited time resolution in the past has been the degradation ("spreading") of the blanking pulse, the signal that effectively turns the electron beam on and off by deflecting it into and out of an aperture in a beam stop. It is an object of the present invention to reduce blanking-pulse degradation. It is a further object of the present invention to increase time resolution in electron-beam instruments.